Inhomogeneous surfaces as selective solar absorbers

Abstract

An outstanding problem facing solar thermal collectors is the development of efficient, cheap and long-lived selective solar absorbing surfaces. Among the best products currently available are complicated inhomogeneous mixtures of metal and dielectric particles optimized from serendipitous discoveries rather than premediated constructions. We have utilized a computer code which uses mean field approximations and can accurately describe the reflectance of inhomogeneous surfaces. This approach gave us the freedom to use realistic materials parameters and surface topologies and arbitrary angles of incidence in our calculations. As a result of calculations on a large number of specific surface configurations and material combinations, we found that the optical properties of such surfaces are largely independent of the details of their topologies and can be described with only two parameters: surface depth, D, and correlation length, 2 a. A main conclusion of the paper is that the impression of a rough surface on an otherwise shiny metal leads to a sharply defined low reflection window between λ II≈20 A and λ≈6 D if λ II< λ I. For λ < λ II, the reflective approaches the bulk value. For λ < λ II, the reflectivity is diffuse. The calculations provide a new insight into the operation of the currently popular selective surfaces and define a bench mark against which their performance can be evaluated.